1. Field of the Invention
The present invention relates to a speed measuring apparatus for measuring the speed of a moving-target relative to the speed measuring apparatus, based on a frequency of a Doppler signal component selected from receiver signals corresponding to acoustic waves reflected from the moving-target.
2. Discussion of Background
Conventionally, a speed measuring method of this kind is theoretically known, which can be conducted by the steps of transmitting toward a moving-target an acoustic wave generated based on a reference signal with a predetermined frequency, receiving an acoustic wave reflected by the moving-target, of which frequency is changed from that of the transmitted acoustic wave by the Doppler effect, and determining the speed of the moving-target based on a change in the frequency of the transmitted acoustic wave.
However, in order to measure the speed of a moving-target which is small in size or is positioned far away from the observer, it will be necessary to implement countermeasures of increasing the transmission level of the acoustic wave which is generated based on the reference signal, and/or increasing the amplification degree of an acoustic wave receiving circuit.
However, when such countermeasures are implemented, there is the risk that normal signal processing cannot be performed. This is because it may occur that an acoustic wave with the same frequency as the frequency of the reference signal, which has a greater level than that of the acoustic wave reflected by the moving-target, travels from a wave transmitting unit and directly enters a wave receiving unit by diffraction, or the wave receiving unit receives reflection waves from fixed objects, which are positioned near the moving-target and are not subjected to the Doppler effect, whereby signal processing circuits such as amplifiers and/or mixers of the wave receiving circuits are saturated.
FIG. 12 is a block diagram of an example of a speed measuring apparatus that can be constructed based on the above-mentioned conventional speed measuring theory.
FIG. 13 is a diagram in explanation of the dynamic ranges, namely the permissible input ranges, of a pre-amplifier and a mixer for use in the example shown in FIG. 12.
The example shown in FIG. 12 is provided with an ultrasonic wave transmitting section 10 comprising a transmitting unit 11, an ultrasonic wave receiving section 20 comprising a receiving unit 21, and a signal processing section 30.
The ultrasonic wave receiving section 20 further comprises a pre-amplifier 22 and a mixer 23 in addition to the receiving unit 21.
As shown in FIG. 13, when a voltage amplification degree (gain) of the pre-amplifier 22 is 200 times, a noise level of the mixer 23 is 2 mV, and a saturation level of each of the pre-amplifier 22 and the mixer 23 is 1 V, the mixer 23 has a dynamic range (DRmix) of 2 mV to 1 V, and the pre-amplifier 22, viewed from the side of the mixer 23, has a relative dynamic range (DRamp) of 10 .mu.V to 5 mV. Therefore, a lower limit of an input level of the pre-amplifier 22, by which an ultrasonic wave reflected by a moving-target and subjected to Doppler shift can be processed, is 10 .mu.V, while the permissible input level of the pre-amplifier 22 for the ultrasonic waves which travel from the transmitting unit 11 and directly enters the receiving unit 21 and for the ultrasonic waves reflected by fixed objects is less than 5 mV.
In this example, if a gain Gamp of the pre-amplifier 22 is increased, for example, to 400 times, the permissible input level of the pre-amplifier 22 is reduced to a half, that is, to less than 2.5 mV, so that it is necessary to implement some countermeasures, such as reducing the level of the ultrasonic wave emitted from the transmitting unit 11 to a half, or reducing the direct entering of the ultrasonic wave from the transmitting unit 11 to the receiving unit 21. Therefore, it is considered that it is extremely difficult to measure the speed of the moving-target which is small in size or is positioned far away from the observer.
Furthermore, in the case where there are foreign objects near the moving-target of which speed is to be measured, and the foreign objects are moving at different speeds from that of the moving-target, it is extremely difficult to measure only the speed of the moving-target, since Doppler-shifted acoustic waves are reflected not only by the moving-target, but also by the foreign objects. For instance, when the speed of a ball thrown by a pitcher in baseball is to be measured, a Doppler-shifted acoustic wave is reflected by the ball itself and various Doppler-shifted acoustic waves are reflected by various parts of the body of the pitcher, and those Doppler-shifted acoustic waves are mixed and received, so that it is extremely difficult to measure only the speed of the ball.